The present invention relates to an apparatus for holding a printing medium on a rotary drum and an ink jet printer using the same.
As personal computers have widely been marketed, their associated color printers are demanded for commercial use. Such conventional color printers are classified into serial, parallel, and line types depending on the mode of printing equipment.
A color printer provided with serial printing equipment includes a printing head having a plurality of ink jet nozzles for ejection of different colors (namely, yellow, magenta, cyan, and black). A conventional color printer provided with the serial printing equipment permits a printing medium such as a sheet of paper of a given size to be conveyed at equal intervals of a pitch in a predetermined direction. During the conveying, the printing head performs reciprocating motions over a surface of the printing medium at a right angle to the conveying direction of the printing medium. The printing head while traveling over the printing medium applies jets of specific inks to the surface of the printing medium at a given location in the reciprocating motion. As the printing head repeats application of the inks to the surface of the printing medium along the conveying direction, a desired image of the inks (which may include characters, numerals, symbols, etc.) is printed in a given area on the surface of the printing medium. The construction of such a conventional color printer provided with the serial printing equipment is well known. The conventional color printer has a printing head which can easily be fabricated and its overall arrangement is relatively simple, thus minimizing the size and lowering the cost. However, the conventional color printer has some disadvantages that the printing head is slow in the speed of printing action and produces a considerable degree of noise, hence being hardly suited for business use which requires production of a large number of prints in a shorter duration of time with less sounds.
A conventional color printer provided with parallel printing equipment allows a printing medium such as a sheet of paper of a given size to be conveyed at a specific speed in a predetermined direction under a plurality of printing units which are arranged at intervals of a given distance along the conveying direction. The printing units are parallel to each other extending at a right angle to the conveying direction between both sides of the printing medium. While the printing medium is conveyed at the specific speed in the conveying direction, different colors (namely, yellow, magenta, cyan, and black) are applied by their respective printing units to print an image on the printing medium. Each of the printing units comprises a photosensitive drum and a static charger, an exposer, a toner developer, a transfer device, a cleaner, and a discharger mounted about the photosensitive drum. The printing unit of this arrangement is known as used in a plain paper copier (PPC). The conventional color printer is quiet during the printing action and higher in printing speed thus producing a large number of prints within a shorter period of time and can thus be suited for business use. However, the conventional color printer provided with the parallel printing equipment includes two or more of the printing units which are expensive and its construction is not simple, thus increasing the cost of production as well as the overall size.
A color printer provided with line type printing equipment also permits a printing medium such as a sheet of paper of a given size to be conveyed at a specific speed in a predetermined direction under a plurality of printing heads which are arranged at intervals of a given distance along the conveying direction. The printing heads are parallel to each other extending at a right angle to the conveying direction between both sides of the printing medium. Each of the printing heads includes a plurality of ink jet nozzles for ejection of one of different color inks (namely, yellow, magenta, cyan, and black). The ink jet nozzles on the printing head are aligned in a row extending at a right angle to the conveying direction between two sides of the printing medium. While the printing medium is conveyed at the specific speed in the conveying direction, the colors are applied by their respective printing heads.
As compared with the serial printing equipment, the line type printing equipment has the following advantages and disadvantages.
The printing head in the line type printing equipment has more ink jet nozzles than that in the serial printing equipment and is thus very expensive. The line type printing equipment allows its printing heads to remain stationary to print a desired image on the printing medium which is conveyed and will thus be faster in the printing action and less noisy than the serial printing equipment.
As compared with the parallel printing equipment, the line type printing equipment has the following advantages and disadvantages.
The line type printing equipment has simpler printing heads in construction than those of the parallel printing equipment thus the overall dimensions is small and the cost of production is low. Also, the printing speed of the line type printing equipment is equal to that of the parallel printing equipment. The line type printing equipment is however lower in resolution of prints on the printing medium than the parallel printing equipment.
Recently, for minimizing the overall size without sacrificing the printing speed, the color printer provided with the line type printing equipment is equipped with an improved device for conveying the printing medium.
Any conventional color printer including the line type printing equipment which are more expensive than that with the serial printing equipment but less expensive than that with the parallel printing equipment is equal in printing speed, smaller in overall size, and slightly lower in resolution of prints than that with the parallel printing equipment, and therefore, it is now common for both business and personal uses.
For minimizing the overall size of a conventional color printer provided with the line type printing equipment without decreasing the printing speed, Jpn. Pat. Appln. KOKAI Publication No. 57-174285 and Jpn. Pat. Appln. KOKAI Publication No. 6-218947 disclose a device for conveying the printing medium that includes a rotary drum having an outer surface thereof facing a plurality of printing heads of the line type printing equipment and a printing medium holding device for detachably holding the printing medium to the outer surface of the rotary drum with certainty. In action, while the rotary drum is rotated a number of times with the printing medium detachably held to its outer surface by the printing medium holding device, the printing heads print down a desired image of different color inks on the printing medium.
However, the printing medium holding device of the prior art has a printing medium holding finger or hook which is constantly projected from the outer surface of the rotary drum for securely holding and releasing the printing medium. The printing medium holding hook however prevents the rotary drum from increasing the speed of rotation or printing action and may also disturb the action of the printing heads over the outer surface of the rotary drum thus discouraging improvement of the resolution of images printed on the printing medium.